"M & E" stands for Membrane and Electrode. A M & E is a structure composed of an ion exchange membrane having a plurality of electrically conductive, catalytically active particles present on one, or both, surfaces of an ion exchange membrane. The electrically conductive, catalytically active particles serve as a particulate electrode when the M & E is used in an electrochemical cell. M & E structures are sometimes called solid polymer electrolyte structures or SPE structures.
"M & E cells" are electrochemical cells employing an M & E structure. Such cells can be operated as an electrolytic cell for the production of electrochemical products, or they may be operated as fuel cells for the production of electrical energy. Electrolytic cells may, for example, be used for the electrolysis of an alkali metal halide such as sodium chloride or for the electrolysis of water.
M & E cells are rather well known in the art and are discussed in detail in the following patents: U.S. Pat. Nos. 4,293,394; 4,299,674; 4,299,675; 4,319,969; 4,345,986; 4,386,987; 4,416,932; 4,457,822; 4,469,579; 4,498,942; 4,315,805; 4,364,815; 4,272,353; and 4,394,229.
In M & E cells, frequently, a gaseous product is produced at the catalytically active particles. The gas bubbles off the catalytically active particles (serving as an electrode) into an electrolyte that contacts the M & E during cell operation. However, the gaseous products that are produced within the pores of the catalytically active particles, or at the catalytically active particle/membrane interface, must diffuse out through the pores of the catalytically active particles before they can bubble into the electrolyte and be removed. Because gases are produced faster than they can escape, they build up within the catalytically active particles or build up at the catalytically active particles/membrane interface and cause a decrease in the efficiency of the operation of the M & E cell. Even worse, some gases permeate the membrane and contaminate the product(s) produced on the other side of the membrane. In chlor-alkali cells, where hydrogen is produced on one side of the membrane and chlorine is produced on the other side of the membrane, hydrogen can permeate the membrane and contaminate the chlorine, or vice-versa. This contamination can be hazardous because of the explosive nature of a chlorine/hydrogen mixture.
The prior art has attempted to minimize the problem of gaseous buildup at the M & E by making porous electrodes for M & E's. See, for example U.S. Pat. No. 4,276,146. Some porous M & E electrodes may be formed by including a pore former, such as sodium chloride, in the catalytically active particles during the M & E preparation process. The sodium chloride is later leached out leaving a porous M & E structure. However, such coatings do not solve the gas diffusion problem because a significant reduction in the hydrogen contamination of the chlorine is not realized. Also, the porous catalytically active particles are fragile, and does not hold up well to the action of gas evolution, which leads to a loss of M & E catalytic results.
The present invention provides an M & E which is especially designed to minimize the permeation of gaseous products through the membrane and into the opposing side of the cell, and to improve the electrical efficiency of the cell.
M & E electrode coatings are made using materials that are rather expensive. It would be advantageous to reduce the amount of material used in the M & E electrode without sacrificing the catalytic activity of the coating. The present invention provides a M & E having substantially less catalytic material without sacrificing the catalytic activity of the coating.
In M & E's of the prior art, a woven, window-screen electrically conductive screen was used to support the M & E. However, window screen is not entirely satisfactory because of its uneven surface. When window-screen type electrically conductive screens are pressed into catalytically active particles or the membrane film, some portions of the window-screen penetrates further than other portions of the window-screen. This causes an uneven contact of the window-screen with the electrode and membrane, resulting in uneven transfer of electrical energy across the face of the membrane. In addition, some parts of the membrane are more likely to rupture when the window-screen penetrates the membrane.
Another problem with the use of window-screen electrically conductive screens concerns the protection of the membrane from being torn or ruptured by mattresses (resilient devices) which may be used to hold a current collector against the catalytically active particles on the surface of the membrane. Window-screen does not provide as much protection as is provided by the substantially flat electrically conductive screens used in the present invention.
The present invention provides a support structure for M & E's which minimize most of the problems with using a window-screen type structure.